1. Field of the Invention
The present invention relates generally to motor current feedback measurements, and relates more specifically to a computational reconstruction of motor current obtained through measurement of a DC bus current feeding the motor driver inverter.
2. Description of Related Art
Inverters for three phase motor drives are well known in the industry. Typically, a DC bus supplies switched power to different phases of an AC motor through an inverter. A design approach used to supply switching commands and sequences to the inverter involves the use of space vector modulation using pulse width modulation (PWM) arrangement. For example, a switch vector plane is illustrated in FIGS. 1A and 2A with specific switch states for controlling the switches of the inverter as noted at the vertices of the hexagon. As known to those of skill in the art, the switch state generally refers to the states of the high side switches in the three inverter half bridges. For example, state V2 (110) means that the high side switches connected to phases U, V and W are controlled so that the high side switches connected to phases U and V are on and the high side switch connected to phase W is off. Correspondingly, the low side switches for phases U, V and W would be, at the same time, respectively, off, off and on.
With this type of motor control, it is desirable to accurately measure motor phase current to provide a high performance control. However, it is often difficult to accurately measure motor phase current over wide current and temperature ranges. For example, current sensing resistors may be used in the motor phase lines. However they do not provide the accuracy and precision needed in a high performance motor drive, especially at high currents, and also waste power. Current sensing transformers may also be used, but these are also subject to non-linearities and can be large and expensive to implement.
In a pulse width modulated (PWM) inverter drive system, motor phase current can be determined from measurement of the DC bus current when non-zero basic vectors are used. Each basic vector is assigned a specific time in a PWM cycle to generate the command voltage vector. However, if a basic vector is used only for a very short period of time, motor phase current cannot be directly determined from the DC bus current. This lack of observability of motor phase current is due to practical considerations in the implementation of the PWM inverter drive system. For example, time delays caused by A/D converter sample and hold times, slewing of voltage during switch turn on, and other delay factors and other parasitic effects that distort the DC link current from an ideal step waveform to an overshoot with ringing type waveform prevent the effects of basic vectors used for a very short time from being observed on the DC bus.
In the space vector plane shown in FIG. 1A, the non-observable regions are illustrated as being located along the borders of the sections of the space vector plane, i.e., around the basic vectors in the shaded regions. Without being able to observe motor phase currents during these control periods, it is difficult to achieve a robust and high performance motor drive. FIG. 1B illustrates the case where one phase can be accurately sampled during vector (100) duration), but neither of the other phase currents can be directly sampled because the duration of the active vector is too short in the interval T2/2, i.e., the reference voltage vector (110) is in the non-observable area of FIG. 1A.
With respect to observability of motor phase current through measurement of the DC bus current, various attempts have been made to overcome the drawbacks described above. In one approach, the switching frequency of the inverter is varied to avoid the problem with unobservability. However, this approach includes a change in the controller gains, and a coordination with the switching frequency, adding to the complexity of the control. Also, when the voltage vector is in the non-observable region, double references in an opposite phase are used to compensate for the non-observable current, increasing the number of switching events.
Another difficulty in observing motor currents by measuring DC bus current is illustrated in FIG. 2A, where a low modulation index is used. During low modulation index cycles, the PWM signals for the three phases have nearly equal durations. As a result, the particular active voltage vector is not used long enough to ensure proper sampling of the DC link (or bus) current. This problem is illustrated in FIG. 2B, where switching intervals occur so closely together that reconstruction of the motor current by measurement of the DC bus current is problematic at best. In addition, a minimum time delay is often used to secure a good current sample of the DC bus current after a particular switching sequence occurs to avoid switching transient signals. The minimum time delay adds to the difficulty of measuring the DC bus current, since there is not enough time between the differing switching states to obtain a current sample for at least one of the phases of the two that are measured, as illustrated in FIG. 2B. Accordingly, accurate motor phase current reconstruction cannot be properly achieved.
When the modulation index is low, resulting in short active voltage vectors that prevent proper sampling of the DC link current, one solution is to replace all active vectors less than a certain time period, 30 microseconds for example, with a zero vector, and then adding the missing time in the next switching period. This technique, however, results in poor performance at low speed due to high harmonic distortion.
Another technique reduces or eliminates zero vectors when the active vectors are too short, and introduces two complementary voltage vectors closest to those already used in the switching cycle. This technique introduces additional switching, and therefore generates more EMI noise. In addition, a very low modulation index still creates problems in performance.
Another technique for overcoming the above drawbacks is to adjust the duty cycle so that switching states are offset to permit an elimination of voltage error within a single switching period. Another technique uses two switching periods in the PWM control to adjust the duty cycles and remove voltage error between the switching events. While these techniques permit the PWM drive to operate at low speed, a higher sample rate is needed to avoid phase errors between the samplings of the motor current.
In a PWM inverter drive system, switching sequences cause current values delivered to the motor phases to ramp up or down over a given switching interval. In practice, it is not feasible to obtain a complete measurement of the current delivered in the motor drive system, due to the high speed processing that would be required. Accordingly, current values are sampled over a given switching interval, potentially resulting in large variations between estimated current values based on samples, and actual current values experienced by the motor. If a current sample taken during a ramp interval is used to estimate the changing current for the entire interval, inaccuracies will occur. It would be desirable to obtain a more accurate technique for sampling motor current based on switching sequences.
Another issue that arises in motor current sampling is transients or “ringing” in the DC bus current when switch transitions occur. An example of problems with ringing is illustrated in FIG. 3. If a sample of DC bus current is taken near a switching event where ringing occurs in a phase current, the sample may not accurately reflect motor phase current. In addition, the DC bus current is typically observed in the shape of a ramp when a switch transition occurs to energize a given phase. If a current sample is taken in an arbitrary part of the switching sequence where the motor phase current is ramping up, the sample DC bus current will not accurately reflect the average motor phase current value over the entire switching interval.